The CuAAC ‘click reaction’ (CuI-catalysed [3+2] alkyne azide cycloaddition) has been used in a vast range of applications due to its high efficiency and orthogonality with almost all other functional groups and solvents. It has found favour in the nucleic acids field as a method of joining together single strands of DNA, cross-linking complementary strands, cyclising single and double strands, labelling oligonucleotides with reporter groups, attaching DNA to surfaces, producing analogues of DNA with modified nucleobases and backbones, synthesizing large chemically modified RNA constructs and creating biochemically active PCR templates. However, CuI has undesirable cytotoxicity even at low concentrations so it is not fully compatible with in vivo applications.
Unfortunately the uncatalyzed alkyne-azide cycloaddition (AAC) reaction, in particular the uncatalyzed DNA-templated AAC reaction with terminal alkynes, is exceedingly slow in comparison unless highly activated alkynes are used, and these are unstable in water.